A time-honored and research-tested way to extend an animal’s lifespan is to restrict its caloric intake. Studies repeatedly confirm that if, say, a lab mouse normally gets two full bowls of lab chow a day, limiting that mouse to one and a half bowls of lab chow a day will make that mouse live longer than the mouse eating the full two bowls. Cool, cool, a longer life is great and all, but what about the downsides of straight calorie restriction, aside from willfully restricting your food intake, ignoring hunger pangs, relegating yourself to feeling discontent with meals, and counting calories and macronutrients obsessively? Are there any others? Sure:

Loss of bone mineral density. Humans who calorie restrict in studies also show signs of lower bone mineral density when compared to humans who lose weight from exercise, particularly in the hip and spine – the two areas most susceptible to fall-related bone breaks. I wrote about this study some time ago here.

Oh, and there’s the fact that the act of restricting one’s calories can be mind-numbing, miserable, and difficult for a great many people, especially if it’s a lifelong pursuit. (Unless, of course, you eat according to the Primal Blueprint and are fat-adapted. It can make CR not only tolerable, but a cinch because we become so good at living off stored body fat. We don’t suffer from sugar lows when we skip meals the way most people who fast do, but I digress.) That’s kind of a biggie.

What about fasting? In previous installments of this series, I’ve explained how fasting can sometimes be described as a “short cut” to the benefits of calorie restriction, an easier (and even more effective) path to the same destination. Studies on fasting/calorie restriction and cancer find that fasting is more effective in a shorter amount of time (weeks or months versus mere days). Does the same hold true for longevity? Can fasting also extend lifespan without making us look like a calorie-restricted monkey?

1945 marks the first real study (PDF) of the effects of intermittent fasting on lifespan in animals. Beginning at day 42 of their lives, rats were either fasted one day in four, one day in three, or every other day. All fasted rats, save for the females who were fasted one day in four, lived longer than control rats on a normal schedule. Although females outlived males in general (like always), fasting had the greatest effect in males. Male rats did best on every other day fasting; female rats did best on one day in three fasting. Fasted rats weighed less than control rats, so they likely also ate less, even though feeding days were ad libitum.

Another study from 1983 showed that rats fed every other day lived longer and had lower body weights than rats fed daily and ad libitum. Fasted rats were less active in their youth, but more active than control rats when adults. The lower body weights in the fasted rats indicate a reduced calorie intake.

In 2000, female mice who fasted for four consecutive days every two weeks lived for 64 weeks on average, while normal fed mice lived just 47 weeks. Interestingly, the fasted mice were heavier than the fed mice throughout the experiment, which indicates that calories weren’t significantly restricted.

Okay, so it looks like fasting promotes longevity, which may be mediated by a reduction in calories. Provided the faster doesn’t gorge him or herself on the feeding days so much that they make up for all the missing calories, it should be effective. In other words, fasting promotes longevity in all but the most ardent of big eaters. I don’t see this as a “gotcha” or a problem, because fasting almost invariably produces calorie restriction. (I might eat a fairly big meal after a long fast, but I definitely have never doubled my intake to make up for the fasted day.) In fact, that fasting makes calorie reduction painless and inadvertent is a highly-touted feature of the practice, and a big part as to why it’s so effective for people who have failed with traditional calorie restriction.

So, how’s it all work? Is it just the calorie restriction?

Perhaps. One potential pathway by which both fasting and CR increase lifespan is via inhibition of mammalian target of rapamycin, or mTOR. The mTOR pathway can be said to drive the aging process. It is necessary for cell growth, like muscle cells (in stable mTOR states) or certain cancer cells (in overactive mTOR states), and it is highly sensitive to nutrient availability and hormonal signaling. In mice, feeding rapamycin – a potent inhibitor of mTOR – increases lifespan. Both not eating (fasting) and lowering potential hormonal messengers (like insulin) also inhibit mTOR activity by spiking AMPK. Indeed, in the presence of insulin, mTOR is upregulated. We know from previous posts that fasting reduces fasting insulin. In insulin resistant people, insulin is chronically elevated and mTOR is overactive. We also know that fasting reduces insulin resistance and increases insulin sensitivity, thus normalizing or inhibiting excessive mTOR activity. This kind of mTOR inhibition also works with CR, but if adherence is easier, side effects are lessened, and AMPK spiking is greater with total caloric restriction (fasting) than with partial CR, which is the more effective method?

So, it’s starting to look like the longevity benefits of fasting can be attributed to the degree of caloric restriction. That is, fasting is total caloric restriction, while CR is partial. When you fast, you’re going whole hog. You’re subjecting yourself to an acute stressor, getting the hormetic benefits, and then recovering from that stressor by eating normally thereafter (until you do it again). When you calorie restrict, you’re undergoing a chronic stressor. Day in, day out, you’re worrying about food, restricting energy and nutrient intake, and there’s really no period of recovery. You’re always residing in a partially restricted state, drifting from paltry meal to paltry meal. There is no feast. It’s like lifting heavy and sprinting a few times a week versus jogging a 10k every day. Chronic cardio versus acute, high intensity exercise.

What about “healthspan”? Can fasting compress morbidity – can it help us in our quest to live long and drop dead? I mean, let’s face it: who wants to be a frail, skinny-fat centenarian relegated to the bed or the walker or the wheelchair?

Well, we know that intermittent fasting appears to conserve more lean mass than CR. In a recent meta-analysis, one group of researchers directly compared studies on calorie restriction to studies on intermittent fasting and found that while both were good for losing weight, “intermittent CR may be more effective for the retention of lean mass.” And at the very least, I’d suggest to you that having better insulin sensitivity, less fat mass, more lean mass, a better-regulated mTOR pathway, improved blood lipids, better glucose control, and a less restricted diet makes for a better healthspan, a more enjoyable life, and a reduced risk of dying from the diseases of civilization.

Bottom line: fasting may not work by some magical pathway separate from caloric restriction. It may, but it hasn’t been established. What we do know is that fasting (whether by inadvertent, enhanced calorie restriction or whatever else) improves lifespan in lab mammals and improves various health markers associated with aging and longevity in both humans and animals. Fasting may not give you an immediate “Life + 25″ boost, and there haven’t been any real lifespan and fasting studies done on humans (if only we had mice-like lifespans!), but if it makes you less likely to get obese, diabetes, heart disease, or cancer, you’re less likely to die from those things. The fewer things you have trying to kill you, the longer you generally live.

That’s it for today, folks. Thanks for reading. Any questions? Comments?

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Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

I think the experience/signal of the fasting in itself generates, up regulates, some biologicals processes (autophagy for instance) that otherwise (including mild CR) would remIn dormant for our complete lifespan. There’s something specif to the act of fasting in itself to improve the health.
Every known religion on earth has its prescription of fasting. Why? …

That’s an interesting thing to point out. I have a mild latex allergy where I get a rash when I inadvertently come in contact with latex – i.e. I wore a respirator at work yesterday and didn’t think that the bands holding the mask to my face had latex in them in I started breaking out in hives later.

But on the interesting side of things, I’ve been on a 24 hour fast at this same time and my rash has been clearing up much faster than normal. I’ll have to try again next time and see if there is a trend.

Interesting, haven’t really tried IF yet.. At the most i’ve skipped lunch a few days out of the week. Doesn’t hamper me one bit, although i’m mad hungry at night. Curious to see how a 16 hr fast might turn out.

I want to try intermittent fasting (16 hr fast, 8 hr window) but i am a serious athlete that plays tennis almost everyday. Will my energy levels be lower or will they increase? Usually if i eat alot before playing, i feel sluggish and tired so hopefully fasting will change that for me.

As long as you’re eating enough after the fast, you’ll be fine. For some people, it might take a few tries to get use to. When I was younger and a lot more active, I couldn’t work out without eating a full on 800+ calorie breakfast. Now, I prefer to workout on an empty stomach. But I will say that the first few times I tried, I was a crab

Just make sure to make up those restricted calories later after the fast, especially as a competitive athlete. If you’re restricting too many calories over the short term, your game will suffer because you won’t have the energy.

If you’re nervous, why not trying IF on a reduced basis? Like maybe once or twice a month, and seeing how that goes. If your bone density isn’t affected too much, maybe you can then try ramping things up?

A recent issue of Harper’s Magazine has an article about fasting, and the recent American history of medical studies of fasting. It’s fascinating and I recommend it highly. The short: fasting seems to differ from caloric restriction over a prolonged period. Seems muscle loss occurs for only a day or two into fasting and then the body converts to ketone energy sources.

My wife and I (for the 10th time in 6 months) began eating primal again about 10 days ago. Using Ketostix (not sure of their reliability) we determined we were in ketosis for the last week or so. Usually takes me 3-4 days to get in to the state. We decided yesterday afternoon after reading Mark’s 2nd post on IF to try an 18 hour fast. So our last meal was around 8pm…woke up this morning…I did feel a bit empty and nauseated in the shower. So I had a big cup of black coffee with two dollops of coconut cream and a bit of Stevia and the discomfort soon passed. Anyway we worked from our home office all day without much thought of food and went on an hour walk at a good pace around 1pm (17 hour mark) and broke our fast at 2pm. We enjoyed some bacon, boiled eggs and fresh blackberries. I felt great all day, mentally clear and had quite a bit of energy. For dinner we had scoconut curried chicken and spaghetti squash around 8pm.

So we’ve decided to keep this routine for as long as we can…does anyone have any constructive criticism to give? Is everyday too often if we are feeling good?

I’d say it largely depends on how much you’re eating when you break your fast and if you’re trying to lose a lot to weight or not.

If you’re calorie restricting too much too fast, it seems you usually bounce back. However, if you’re eating enough at dinner and you’re not getting hunger pains during the day, I can’t see it hurting. I personally know several people that eat only once a day – at dinner – and then several “bird eaters”, that munch all day. I think it largely depends on how you feel and there’s no right nor wrong answer.

Personally, my eating is all over the place. Sometimes I’m in the groove at work and will go up to 36 hours on a cup of coffee and maybe some cream. Other days, I feel like I can’t eat enough.

I guess I just mean to say that you’ll eventually reach homeostasis; how you do it is irrelevant, just don’t be afraid to try a several different things.

The only wrong answer is expecting that there is one hard-and-fast way to do it

I’m still not at the point where I’m the best fast-er in the world. A lot of days I get off to a good start, but by mid-day, I’m starved. A lot of times I’ll steal like 10 dark chocolate covered peanuts from the candy bowl, and then be able to resume “fasting” for the rest of the day until my evening meal, but I still feel like I it wasn’t much of a fast then, in the long run

I may be mistaken here and I am far from an expert, but I was always under the impression that if you were to fast your body effectively went into ‘starvation’ mode and would retain fat at a high level from the next meal you eat.

I used to be on the “small, frequent meals” bandwagon because of its supposed “metabolic boost” and “starvation mode.” Science doesn’t back the claim, however. Short-term fasting actually increases metabolism and the earliest evidence for a decrease in metabolic rate is at 60 hours.

I’ve tried fasting from 8:00pm to 3:00 the next day, but have never made it past 10:00am – I get too hungry. Should I just push on through that? Part of me hates being hungry but I know it’s just another sensation. Have people found that drinking alcohol the night before affects how hungry they are the next day and their ability to fast?

Try fasting when you haven’t had alcohol the night before and have a good protein feed as your last meal. Hunger is not easy to deal with if you are not used to it but believe me, as I said above, if you can just stretch the fast out a little bit further and breathe through the discomfort you will feel better for trying. And don’t beat yourself up if you can’t. Just don’t give up trying. Good Luck

Okay, so this fasting thing is looking more and more appealing. I am still confused on how to do it though. I was thinking the “eating window” type of fast looked the easiest… so do you have a restricted eating window everyday forever, or do you have it 1-2X a week? Also, how does this work for an exclusively breastfeeding mama?

There is some evidence of longevity in monks, in particular, Carmalite monks and nuns who only eat one meal a day as part of their routine have a longer life expectancy than the general population. hough hat may also be related to stress free lives… if getting up an regular intervals during the night is stress free!

I Have been on PB for 3 months. I did my first 36 hour fast which ended yesterday with supper. Dit not feel hungry at all, didnt get any energy dips. When supper came along I was as hungry at supper as before PB when I had Lunch

Supper was a Club steak and salad, I was Stuffed, felt like I ate to much. It feels as if it was an appetite reset for me.

I stumbled upon your website as I was looking for information about what exactly was a “low-carb” lifestyle. Was is 50? 100? 300? And so I discovered your site. I have to say that I am now an avid reader of your site. I have a science background, and am currently a photographer by trade. I love how your articles are often backed by real scientific research…published research. I am skeptical, and like to find out all I can about a particular topic before I jump on the bandwagon. I also need to weigh if a particular “diet”/way of living is sustainable for me. With that said, I started on a lower carb path after Thanksgiving. I had come across the book “Wheat Belly”, and decided to “give it a try”. My appetite naturally decreased, I no longer has insatiable cravings for all things carbs, and I lost 15 pounds that first month, when all of my friends inevitably gained the same amount due to the holidays. Seeing the results firsthand, I decided to give it another month. I can happily say I have been wheat-free for 4 months now. It is no longer a diet, it is just how I eat. It is normal for me. I lost another 10 pounds, and have been in a plateau for weeks. (I am 5’4″ tall woman and currently weigh 208). I started adding walking exercise and more overall activity to my daily life, and I am starting to add aspects of Primal living to my life as well. I really like this idea of fasting. Over the years I have done several fasts for Christian religious reasons. The most recent was a “Daniel Fast” that lasted 3 weeks. I felt incredible while doing it. I like the idea in this article that fasting is not about starvation, but rather a way for your body to turn to burning it’s own fat. I am trying out an ADF method for the next few weeks. I lost 3 pounds in the first 3 days that I couldn’t seem to get past. I feel great, even on fasting days, and relly think I may incorporate this as a way to gain my health back. So thank you for your informative site!

A very timely post! I have a question tho’. I have been following a very low carb diet, with the intention of becoming keto adapted, aka. – a fat burner. After several weeks of consistent very low carbs, reduced levels of ketones appearing pin the KetoStix and good energy levels… I think I have made it to keto adapted!!

So, today I tried my very first IF. As per Mark’s suggestion on the website, I decided to try a compressed eating window. So, I ate my last snack of 2 TBSP of almond butter at 8 pm… then did not eat again until 11 am the next morning.

Just for a lark I decided to test my blood glucose to see just how ‘in the basement’ ir would be after a 14 hour total fast. I was surprised to see my BG at 5.4 mmol/l. I am from Canada, so I believe this would translate to 97.2-ish. This seems high to me for a healthy person (no diagnosis of any problems, anyway).

Is this an indication that my body may have created glucose on it’s own? Is this a concern? Any thoughts would be appreciated!

What about fasting before you are an adult? At what point would it be good to instill this sort of habit in children? Obviously growing and energy requirements are different through our teens…should something like this wait until your 20’s…or?

Would eating only one meal a day, dinner, be considered fasting? Or does it have to span over 24 hours?

For the last 6 months, I’ve stopped having a meal for breakfast & lunch… instead I usually juice some vegetables/fruits or mix some vegetable powder (ex. new chapter berry green) with some water. Dinner and the weekends is usually the only time I’m eating.

I’ve lost 30 lbs in 6 months, and I honestly have gotten quite used to this pattern.

I have been doing the same since Sept 2010 and for the past few months, all I eat is dinner. My mind is so clear and I am very alert during the day. You should read about the Warrior Diet.
A fast is defined as zero calorie intake. So even though fruits/vegetables break your fast, its not a big deal as long as you dont consume too many calories (undereat/overeat cycles). Since u have been doing this for 6 months, you should be able to clearly distinguish when your body is in a fasted state versus not in a fasted state. If u feel the same in the morning after eating a couple of eggs and bacon vs. eating a fruit or two, I would say you are breaking your fast by eating the fruits

“Interestingly, the fasted mice were heavier than the fed mice throughout the experiment, which indicates that calories weren’t significantly restricted.”

This “fasted mice” were heavier than the fed mice were not noted in the previous experiments. It seems to me the difference was the four consecutive fasting days. Could this induce less activities (or lower metabolism) than the other fasting schedule?

How does fasting play a role with metabolism? I believe my metabolism is slow because I routinely stop eating by 8pm and don’t eat again unti around 11am or later.
I have been told that is a big mistake as my metabolic rate goes down (starvation mode). How do I increase my metabolic rate? I do work out daily. I walk or slow jog 1 mile 3 times a day and practice karate 4 times a week 2 hours.

Has anyone tried carb back loading? Everything is scientifically referenced and for someone who would constantly feel tired on a more primal diet this has made a huge difference! A lot of similarities to low carb, IF but with gaining muscle as the main goal. Just looking for anyone who might have tried it.

I eat not exactly one meal a day but refrain from food until late afternoon/evening and then eat over a two hour period. I have done this for many years along with occasional 4/5 day fast.
I am 66yrs old very fit and active and have a satisfied 36yr old girlfriend.

“…restricting one’s calories can be mind-numbing, miserable…” Its obvious to me you’ve never fasted a day in your life. I fast every 24 hours and I feel like superman during the time I don’t eat. Never a dull and miserable experience for me, and I’ve been at it for 3 months now! I’d recommend try it!

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